1. Field of the Invention
The present invention relates to an arrangement and a method of preprocessing binary picture data before runlength encoding same, and more specifically to such an arrangement and method which features a high data compression efficiency in transmitting binary picture data of a document which includes both monotone and halftone picture images.
2. Description of the Prior Art
Run length coding, viz., the coding of lengths of consecutive black/white picture elements is well known in the art of facsimile transmission for the purpose of transmission redundancy reduction. This technique allows for the compression of binary picture data obtained by scanning a document including black/white images such as letters.
The Modified Huffman code and the two dimensional extension of the same, called the Modified READ code, are examples of international standards established for the purposes of increasing the utility of the above mentioned method.
However, even though the run length coding has found extensive use in redundancy reduction in two-level facsimile digital transmission, the same efficiency is not expected with the image processing of a halftone or multi-tone document such as a photograph. In order to overcome this problem, it has been proposed to generate a prediction error signal using a dithering technique. The prediction error signal is then processed by a convolution circuit which includes a shift register and an exclusive-OR gate. Thus, the number of transition points are effectively reduced.
However, another problem occurs in the event of processing a document which includes black/white and multi-tone images together. That is, according to a known technique, an operation should select one of the two-level and multi-tone transmission modes prior to transmission and, accordingly an operator is uncertain which mode should be selected in the case where the two-level and multi-tone images coexist in the same document to be transmitted.
In order to overcome the problem mentioned just above, another prior art has been disclosed in U.S. Pat. No. 4,918,540 assigned to the same entity as the assignee of the present invention. According to this prior art, the number of one line pixel transition points after being preprocessed (denoted by a first number), is compared with that which has not been preprocessed (denoted by a second number). If the above mentioned first number is smaller than the second number, the one line pixel data with the smaller number of transition points is transmitted. In this instance, a tag signal indicative of which line image data (viz., preprocessed or not preprocessed) is transmitted, should be attached to each line transmission.
However, the aforesaid known technique has encountered the problem in that if the two-level and multi-level images coexists in one line, a high compression efficiency is not expected. Another problem of this prior art is that the transmission time is undesirably increases due to the attachment of the tag signal to each line.